Method and apparatus for transmission power control

ABSTRACT

The invention relates to a system for power control in a cellular communication system. The power control may be operated ( 201 ) in a first mode of operation wherein power control data are determined in response to a quality parameter, such as a desired signal to noise ratio. The method further comprises entering ( 205 ) a reduced power mode of operation by communicating power down power control data between a base station and a communication unit. The base station and/or communication unit then proceeds to operate ( 207 ) in the reduced power mode by communicating power control data corresponding to a reduced transmit power level. The reduced transmit power level may specifically be zero. After a given duration, the base station and/or communication unit proceeds to exit ( 211 ) the reduced power mode by communicating power up power control data between the base station and the communication unit.

FIELD OF THE INVENTION

The invention relates to a method and apparatus for power control for atransmitter in a cellular communication system and in particular to asystem for entering a reduced power control mode.

BACKGROUND OF THE INVENTION

FIG. 1 illustrates the principle of a conventional cellularcommunication system 100 in accordance with prior art. A geographicalregion is divided into a number of cells 101, 103, 105, 107 each ofwhich is served by base station 109, 111, 113, 115. The base stationsare interconnected by a fixed network which can communicate data betweenthe base stations 109, 111, 113, 115. A mobile station is served via aradio communication link by the base station of the cell within whichthe mobile station is situated. In the example of FIG. 1, mobile station117 is served by base station 109 over radio link 119, mobile station121 is served by base station 111 over radio link 123 and so on.

As a mobile station moves, it may move from the coverage of one basestation to the coverage of another, i.e. from one cell to another. Forexample mobile station 125 is initially served by base station 113 overradio link 127. As it moves towards base station 115 it enters a regionof overlapping coverage of the two base stations 113 and 115 and withinthis overlap region it changes to be supported by base station 115 overradio link 129. As the mobile station 125 moves further into cell 107,it continues to be supported by base station 115. This is known as ahandover or handoff of a mobile station between cells.

A typical cellular communication system extends coverage over typicallyan entire country and comprises hundreds or even thousands of cellssupporting thousands or even millions of mobile stations. Communicationfrom a mobile station to a base station is known as uplink, andcommunication from a base station to a mobile station is known asdownlink.

The fixed network interconnecting the base stations is operable to routedata between any two base stations, thereby enabling a mobile station ina cell to communicate with a mobile station in any other cell. Inaddition the fixed network comprises gateway functions forinterconnecting to external networks such as the Public SwitchedTelephone Network (PSTN), thereby allowing mobile stations tocommunicate with landline telephones and other communication terminalsconnected by a landline. Furthermore, the fixed network comprises muchof the functionality required for managing a conventional cellularcommunication network including functionality for routing data,admission control resource allocation, subscriber billing, mobilestation authentication etc.

Currently the most ubiquitous cellular communication system is the2^(nd) generation communication system known as the Global System forMobile communication (GSM). GSM uses a technology known as Time DivisionMultiple Access (TDMA) wherein user separation is achieved by dividingfrequency carriers into 8 discrete time slots, which individually can beallocated to a user. A base station may be allocated a single carrier ora multiple of carriers. One carrier is used for a pilot signal whichfurther contains broadcast information. This carrier is used by mobilestations for measuring of the signal level of transmissions fromdifferent base stations, and the obtained information is used fordetermining a suitable serving cell during initial access or handovers.Further description of the GSM TDMA communication system can be found in“The GSM System for Mobile Communications’ by Michel Mouly and MarieBernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.

Currently, 3^(rd) generation systems are being rolled out to furtherenhance the communication services provided to mobile users. The mostwidely adopted 3_(rd) generation communication systems are based on CodeDivision Multiple Access (CDMA) wherein user separation is obtained byallocating different spreading and scrambling codes to different userson the same carrier frequency. The transmissions are spread bymultiplication with the allocated codes thereby causing the signal to bespread over a wide bandwidth. At the receiver, the codes are used tode-spread the received signal thereby regenerating the original signal.Each base station has a code dedicated for a pilot and broadcast signal,and as for GSM this is used for measurements of multiple cells in orderto determine a serving cell. An example of a communication system usingthis principle is the Universal Mobile Telecommunication System (UMTS),which is currently being deployed. Further description of CDMA andspecifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley &Sons, 2001, ISBN 0471486876.

The frequency band allocated for a cellular communication system istypically severely limited, and therefore the resource must beeffectively divided between mobile stations. A fundamental property of acellular communication system is that the resource is dividedgeographically by the division into different cells. Thus, a certainamount of resource (for example a frequency band) may at a given time beallocated to a given cell thereby reducing the resource allocation toneighbouring cells. In order to optimise the capacity of a cellularcommunication system, it is important to minimise the impact ofinterference caused by or to other mobile stations. An importantadvantage of a cellular communication system is that due to the radiosignal attenuation with distance, the interference caused bycommunication within one cell is negligible in a cell sufficiently farremoved, and therefore the resource can be reused in this cell. Inaddition, the resource is typically divided within one cell and betweencells by division of the resource in the time domain, the frequencydomain and/or the code domain. Different communication systems usedifferent principles for this division. The resource allocation may bestatic or dynamic dependent on the current load of the communicationsystem, and typically a combination of static and dynamic resourceallocation is used.

Hence, it is of the outmost importance for cellular communicationsystems to optimise the use of the available radio resource. However,the allocation of resource to different services and different mobilestations tend to require complex management and control involvingsignificant signalling. This is particularly unsuitable for efficientresource utilisation in the presence of significant fluctuations in theradio environment.

For example, circuit switch calls predominantly seek to cope with severefading conditions by changing the communication parameters to ensure asuitable quality. Specifically, calls may be handed over to neighbouringcells. However, handover is a slow process that requires significantsignalling between base stations and mobile stations. Therefore,repeated handovers must be avoided and consequently handovers are bestsuited to permanent changes in the radio environment and less suited fordealing with short term variations. As another example, packet basedcommunications seek to cope with interference variations bysophisticated scheduling algorithms. However, these algorithms requirecomplex signalling and are therefore unsuitable for short termfluctuations in the parameters of the radio environment.

Hence, an improved system for controlling the radio interface resourceutilisation would be advantageous and in particular a system allowingfor increased flexibility, lower complexity, less signalling overhead,faster operation and/or improved resource utilisation in view offluctuations in the characteristics of the radio environment would beadvantageous.

SUMMARY OF THE INVENTION

Accordingly, the Invention seeks to mitigate, alleviate or eliminate oneor more of the above mentioned disadvantages singly or in anycombination.

According to a first aspect of the invention there is provided a methodof power control for a transmitter in a cellular communication systemcomprising the steps of in a first mode of operation determining powercontrol data in response to a quality parameter of a communicationbetween a base station and a communication unit, and communicating thepower control data between the base station and the communication unit;entering a reduced power mode of operation by communicating power downpower control data between the base station and the communication unit;operating in the reduced power mode by communicating power control datacorresponding to a reduced transmit power level; and exiting the reducedpower mode by communicating power up power control data between the basestation and the communication unit.

The invention allows for a reduced interference from a transmittersupporting a communication between a base station and a communicationunit. The method enables the advantages of low complexity, high speedand/or low overhead. The method is suitable for controllingtransmissions in response to fluctuations in the characteristics of theradio environment. The method enables an improved capacity of thecommunications system as interference from communications temporarilycausing high interference may be temporarily reduced. No signalling inexcess of the power control data need be communicated, and the capacityis thus further reduced by the reduction in signalling data. Althoughthe reduced power mode may prevent or reduce the communication capacityof the communication, the reduced interference is likely to benefitother communications thereby increasing the capacity of thecommunication system as a whole.

According to a feature of the invention, the power control is an uplinkpower control and the power control data is transmitted from the basestation to the communication unit. The method may advantageously beapplied to control the transmit power of a communication unit. Thisallows for the fixed network and base station to control the operationof the communication unit to suit the needs of the communication systemas a whole. A standard communication unit meeting the current powercontrol standards may be used without any modifications. Hence,deployment in existing already deployed communication systems isfeasible without modifications to the communication units. Thecommunication unit need not know that a reduced power mode is entered.

According to a feature of the invention, the power control is a downlinkpower control and the power control data is transmitted from thecommunication unit to the base station. The method may advantageously beapplied to control the transmit power of a base station. This allows forindividual communication units to control the operation of the basestation to suit the current radio environment.

According to a feature of the invention, the reduced transmit powerlevel is substantially zero. This allows for the interference reductionto be maximised and therefore allows for the maximum benefit to othercommunications. It furthermore enables a simple implementation.

According to a feature of the invention, the power control datacommunicated in the reduced power mode is power down control values.This allows for a simple method of reducing the transmit power in thereduced power interval A continuous transmission of power control valueswill for a sufficiently long duration of the reduced power mode resultin the transmit power being reduced to substantially no transmit power.Simply transmitting power down control values removes any requirementfor determining suitable power control values for meeting a givencriterion and thus allows for very simple operation.

According to a feature of the invention, the reduced transmit powerlevel allows a reduced data rate communication between the communicationunit and the base station. This allows for communication to be exchangedwhile reducing the interference caused. Specifically, this may allow forcontrol and signalling information to be maintained during the reducedpower mode.

According to a feature of the invention, the step of exiting comprisestransmitting power up power control data until the transmit powercorresponds to a power level determined in response to the qualityparameter. This allows for the transmit power to quickly be returned toa suitable level for supporting the subsequent communication.

According to a feature of the invention, the step of exiting comprisestransmitting power up power control data until the transmit powercorresponds to a power level corresponding to the power level prior toentering the reduced power mode. This allows for the transmit power toquickly be returned to a suitable level for supporting the subsequentcommunication without requiring that a suitable power control level isfirst determined.

According to a feature of the invention, a duration of the reduced powermode is less than a data retransmission interval associated with thecommunication between the communication unit and the base station. Thisallows for any data lost during the reduced power mode to be recoveredby retransmissions. Specifically, a retransmission scheme may bespecified for the communication and thus re-transmission mayautomatically recover any lost data during a limited duration reducedpower mode operation.

According to a feature of the invention, the method further comprisesthe step of determining that a quality level of the communicationbetween the communication unit and the base station cannot be achieved,and in response entering the reduced power mode. Specifically, if thequality level cannot be attained despite communicating power up powercontrol data for a sufficient duration of time, this may be taken as anindication that the transmitter has reached saturation. This is likelyto be caused by a significant fade condition, which may soon disappear.The quality level may be an error rate, signal to noise ratio and/orsignal to interference ratio. This provides a suitable means ofdetermining that a reduced power mode may advantageously be entered.

According to a feature of the invention, the method further comprisesthe step of determining that a transmit power of the transmitter exceedsa threshold and in response entering the reduced power mode. Thetransmit power may be a total transmit power or a transmit power of aresource unit of the transmitter such as a transmit power per code, percarrier or per communication unit. This allows for an efficient means ofdetermining that the radio characteristics are currently disadvantageousfor the communication, and that a reduced power mode may advantageouslybe entered.

According to a feature of the invention, the method further comprisesthe step of determining that an interference level exceeds a thresholdand in response entering the reduced power mode. Specifically, theinterference level may be an interference level at the base station, atanother base station, at the communication unit and/or at anothercommunication unit. This allows for an efficient means of determiningthat the radio characteristics are currently disadvantageous for thecommunication, and that a reduced power mode may advantageously beentered.

According to a feature of the invention, the method further comprisesthe step of determining that a propagation characteristic exceeds athreshold and in response entering the reduced power mode. Preferably,the propagation characteristic is a path loss of a communication linksupporting the communication between the communication unit and the basestation.

Parameters and characteristics associated with communication betweenbase stations and communication units are typically frequentlydetermined in cellular communication systems. These characteristics mayfor example be associated with an interference level, a path loss, afade condition, a delay spread and/or a noise level. Thesecharacteristics may provide an efficient means of determining that theradio characteristics are currently disadvantageous for thecommunication and that a reduced power mode may advantageously beentered.

According to a feature of the invention, the method further comprisesthe step of determining that a duration of the reduced power modeexceeds a threshold and in response exiting the reduced power mode. Thethreshold may specifically relate to a predetermined, fixed orpseudorandom time interval. Hence, the power control may automaticallyexit the reduced power control after a given duration. This allows for asimple and efficient means of ensuring that the reduced power mode issufficiently short not to have an unacceptable impact on thecommunication and that it can follow the fluctuations in the propagationconditions.

According to a feature of the invention, the method further comprisesthe step of determining that a quality characteristic of a datacommunication between the communication unit and the base station isimproving and in response exiting the reduced power mode. Specifically,it may be determined that a signal to interference ratio, signal tonoise ratio or error rate is improving despite the power control beingin reduced power mode. This may be an indication that the radiocharacteristics are improving and may no longer be unfavourable. Hence,it may provide an efficient means of determining that the radiocharacteristics are currently not disadvantageous for the communication,and that the reduced power mode may advantageously be exited.

According to a feature of the invention, the method further comprisesthe step of determining that an interference level is below a threshold,and in response exiting the reduced power mode. Specifically, theinterference level may be an interference level at the base station, atanother base station, at the communication unit and/or at anothercommunication unit. This allows for an efficient means of determiningthat the radio characteristics may currently not be disadvantageous forthe communication, and that the reduced power mode may advantageously beexited.

According to a feature of the invention, the method further comprisesthe step of determining that a propagation characteristic is below athreshold and in response exiting the reduced power mode. Preferably,the propagation characteristic is a path loss of a communication linksupporting the communication between the communication unit and the basestation. Parameters and characteristics associated with communicationbetween base stations and communication units are frequently determinedin cellular communication systems. These characteristics may for examplebe associated with an interference level, a path loss, a fade condition,a delay spread and/or a noise level. These characteristics may providean efficient means of determining that the radio characteristics arecurrently not disadvantageous for the communication and that the reducedpower mode may advantageously be exited.

According to a feature of the invention, the method further comprisesthe steps of determining an expected interference level for a pluralityof communication units including the communication unit; determining atotal expected interference level; and entering the communication unitinto the reduced power mode if the total expected interference levelexceeds a threshold. This allows for an efficient method of allocatingradio resource such that an interference threshold is not exceeded.

According to a feature of the invention, the power control is operatedin accordance with the 3^(rd) Generation Partnership Project TechnicalSpecification TS 25.214. Hence, an improved method of power control isprovided allowing for increased performance and/or capacity of a 3^(rd)generation cellular communication system.

According to a second aspect of the invention, there is provided: anapparatus for power control for a transmitter in a cellularcommunication system, the apparatus comprising: means for, in a firstmode of operation, determining power control data in response to aquality parameter of a communication between a base station and acommunication unit, and communicating the power control data between thebase station and the communication unit; means for entering a reducedpower mode of operation by communicating power down power control databetween the base station and the communication unit; means for operatingin the reduced power mode by communicating power control datacorresponding to a reduced transmit power level; and means for exitingthe reduced power mode by communicating power up power control databetween the base station and the communication unit.

These and other aspects and advantages of the invention will be apparentfrom and elucidated with reference to the embodiment(s) describedhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will be described, by way of exampleonly, with reference to the drawings, in which

FIG. 1 is an illustration of a cellular communication system inaccordance with the prior art; and

FIG. 2 is an illustration of a flow chart for a method of power controlin accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The following description focuses on an embodiment of the inventionapplicable to a Universal Mobile Telecommunication System (UMTS).Specifically, the description will concentrate on an embodiment whereinpower control is performed in accordance with 3^(rd) GenerationPartnership Project Technical Specification TS 25.214. However, it willbe appreciated that the invention is not limited to this application butmay be applied to many other cellular communication systems includingfor example 2^(nd) generation communication systems such as GSM and GPRSbased communication systems.

The following description will furthermore focus on a descriptionwherein an uplink power control is used to reduce the transmit power ofa communication unit. However, it will be apparent that the invention isnot limited to this application and is for example equally applicable todown link power control. A method of power control in accordance with anembodiment of the invention. will in the following be described withreference to the cellular communication of FIG. 1.

In cellular communication systems, it is imperative to manage the radiolinks 119, 123, 127, 129 between the base stations 109, 111, 113, 115and communication units 117, 121, 125 such that the resource used by agiven communication link is as low as possible. Thus, it is important tominimise the interference caused by the communication to or from acommunication unit 117, 121, 125, and consequently it is important touse the lowest possible transmit power. As the required transmit powerdepends on the instantaneous propagation conditions, transmit powers aredynamically controlled to closely match the conditions. For thispurpose, the base stations 109, 111, 113, 115 and communication units117, 121, 125 operate power control loops, where the receiving endreports information on the receive quality back to the transmitting end,which in response adjusts it's transmit power.

In UMTS, the downlink power control operates by the communication unitreporting the error rate of the received signal, and the base stationadjusting the transmit power in response. Specifically, a base stationdecreases the transmit power, if the error rate is below a desireddownlink error threshold, and increasing it otherwise. In the uplinkdirection, the base stations measure the received error rate and compareit to an uplink error threshold. If the error rate is below the uplinkerror threshold, the base station transmits power down power controldata to the communication unit. If the error rate is above the uplinkerror threshold, the base station transmits power up power control datato the communication unit.

In UMTS, both an inner power control loop and an outer power controlloop is implemented. Inner loop power control operates as follows. Thereceiving entity of a radio link measures the received Signal toInterference Ratio (SIR), and compares it to a locally stored targetSIR. A command is sent back to the transmitter to increase transmittedpower if the measured SIR is less than the target. Conversely, if themeasured SIR is greater than the target, a command is sent to thetransmitter to decrease the transmitted power.

The target SIR is set by a known feature called outer loop powercontrol. Its function is to maintain the frame error rate (ER) of theradio link at or below a given value or threshold. The frame error rateof the received signal is measured by one of a number of knowntechniques, and the SIR target is adjusted to try to ensure that the FERis at or below the given value.

In the preferred embodiment, a base station 111 is communicating with acommunication unit 121 over a radio link 123. The base station 111comprises means for determining power control data. The power controldata is communicated to the communication unit 121 as Transmit PowerControl (TPC) bits in the downlink communication is well known in art.The TPC bits are set to 00 to decrease the transmit power of thetransmitter of the communication unit (power down) and to 11 to increasethe transmit power (power up). TPC bits are transmitted at a frequencyof 1.5 kHz and accordingly the communication unit 121 receives a powerup or a power down power control data values every 0.667 mseconds.Hence, the transmit power of the communication unit 121 is accuratelycontrolled to meet the fluctuating propagation conditions.

In the preferred embodiment, the base station is operable to operate ina first mode of operation wherein power control data is determined inresponse to a quality parameter of a communication from thecommunication unit 121 to the base station 111. In this mode, the basestation 111 operates a power control in accordance with the technicalspecification TS 25.214 Specifically, the quality parameter is a SIR asdescribed above. The determined power control data is communicated tothe communication unit by the downlink transmissions. Specifically, TPCbits are determined and transmitted.

In the preferred embodiment, the base station furthermore comprisesmeans for determining that the current propagation conditions for thecommunication from the communication unit 121 to the base station 111are bad. Specifically, the base station 111 may determine that apropagation loss is unacceptably high.

In response to this detection, the base station 111 enters a reducedpower mode of operation. It will be appreciated that for this embodimentthe reduced power mode refers to the base station 111 controlling thetransmit power of the communication unit to be reduced and does notrelate to a transmit power of the base station 111. The base station 111enters this mode by sending a stream of power down power control datavalues. Hence, all TPC bits are set to 00 for a given duration. Thiscauses the transmit power of the communication unit 121 to besignificantly reduced and the communication unit 123 thus enters areduced power mode of operation.

The base station 111 operates in the reduced power mode by transmittingpower control data which corresponds to a reduced transmit power level.Specifically, only power down control values may be communicated therebyminimising the transmitted power. Accordingly, the interference causedby the communication unit 121 is significantly reduced.

In the preferred embodiment, the base station 111 operates in thereduced power mode for a given time interval after which it exits thereduced power mode. The base station 111 exits the reduced power mode bycommunicating power up power control data from the base station to thecommunication unit. Specifically, the TPC bits are set to 11 for a giveninterval or alternatively or additionally the base station 111 exits thereduced power mode by instantly returning to the first mode ofoperation. As the transmit power is below the required level, thestandard operation of the power control in the first mode will cause thebase station 111 to transmit power up power control data.

Hence, in response to unfavourable propagation conditions, thecommunication unit is entered into a reduced power mode for a shortinterval. Although this may cause loss of data for the communicationunit 121, it also reduces the interference significantly therebyallowing for the communication capacity of the entire communicationsystem to be increased.

In particular for a CDMA based cellular communication system, the radioresource used by communicating a given service is significantly higherfor unfavourable propagation conditions in comparison to favourableones. Therefore, entering a reduced power mode when the propagationconditions are unfavourable may significantly reduce the resource usageby the communication unit 121. In many cases, any data loss during thereduced power mode interval is insignificant. Specifically,communication systems such as UMTS comprise re-transmission schemeswherein any lost data is re-transmitted at a later time.

FIG. 2 is an illustration of a flow chart for a method of power controlin accordance with a preferred embodiment of the invention. The methodis applicable to a cellular communication system such as thatillustrated in FIG. 1. The method will in the following be described inmore detail with reference to the cellular communication of FIG. 1.

In step 201, the base station 111 operates in a first mode of operation.The base station 111 performs a standard power control algorithm inaccordance with TS 25.214 as is well known in the art. Specifically, thebase station determines the uplink SIR and generates power up data ifthe determined SIR is below a preferred value and power down data if itis above the preferred value. The generated power control data istransmitted to the communication unit 121 which increases or decreasesthe transmit power accordingly.

Step 201 is followed by step 203 wherein it is determined whether thebase station 111 should enter a reduced power mode or remain in thefirst mode of operation. If it is determined that the base station 111should remain in the first mode, the method continues by iterating step201. If it is determined that the base station 111 should enter areduced power mode, the method continues in step 205.

It is within the contemplation of the invention that any suitable methodfor instigating the reduced power mode may be used. However, in thepreferred embodiment the base station enters the reduced power modeoperation in response to determining that the conditions for thecommunication between the communication unit and the base station havebecome unfavourable.

In one embodiment, step 203 comprises determining if the transmitter ofthe communication unit exceeds a given threshold. This may in someembodiments be determined by the communication unit periodicallytransmitting information of the current transmit value. Specifically, itmay be determined if the transmitter is in saturation. If so, thetransmit power of the communication unit cannot be increased further.Hence, the caused interference is at a maximum level. This situation islikely to occur for deep fades, and by entering a reduced power mode thebase station may temporarily reduce the transmit power of thecommunication unit thereby reducing the interference.

Alternatively or additionally, step 203 may comprise determining that aquality level of the communication between the communication unit andthe base station cannot be achieved. Specifically, the base station 111may continuously determine the received error rate or SIR. If a giventhreshold cannot be achieved despite continuous transmission of power uppower control data, this is an indication that the transmitter of thecommunication unit 121 is at maximum power, and that the radioconditions are such that an acceptable quality cannot be achieved.Hence, this allows for the base station to determine that a reducedtransmit power mode may advantageously be entered based on only thecommunication signal received from the communication unit 121.

Alternatively or additionally, step 203 may comprise determining that aninterference level exceeds a threshold. For example, the base station111 may continuously measure the total interference level at the basestation 111. If the interference level increases above a giventhreshold, the base station 111 becomes overloaded, and in response itmay be advantageous to temporarily enter a reduced power mode for one ormore communication units. This will temporarily reduce the interference.In many cases, the radio propagation characteristics have changed whenthe communication unit exits the reduced power mode, thereby allowingcommunication without the interference level being exceeded.

As another example, the interference threshold may be related to anothercommunication unit. For example, it may be known that two communicationunits are close to each other, and if the interference level for one ofthe communication units is above a given level, the base station mayenter a reduced power mode for the other communication unit therebyreducing the interference level. This may for example provide a shortreduced interference period that may be used to ensure reliablesignalling to the communication unit, for example to instigate ahandover of the communication unit.

Alternatively or additionally, step 203 may comprise determining that apropagation characteristic exceeds a threshold. For example, in somecommunication systems, the transmit power level of the communicationunit may be communicated to the base station. In response, the basestation may determine the path loss of the communication link as thedifference between the transmitted power level and the received signalleveL If this propagation loss exceeds a threshold, it may beadvantageous to temporarily suspend the communication until the pathloss is reduced and the base station may enter a reduced power mode forthe communication unit.

In step 205, the base station enters the reduced power mode bycommunicating power down power control data to the communication unit.In the preferred embodiment, the base station continually transmits TPC00 bits. This causes the communication unit to reduce the transmit powerregardless of the current quality of the communication. Hence, thecommunication unit similarly enters a reduced power mode.

Step 205 may continue until the transmit power has reached a given levelor for a given time interval guaranteed to reduce the transmit powerlevel to a sufficient level.

Step 205 is followed by step 207 wherein the base station andcommunication unit operate in the reduced power mode. The base stationoperates in this mode by communicating power control data correspondingto a reduced transmit power level. Specifically, the base stationcommunicates power up and power down control messages to meet acriterion that results in a reduced transmit power of the communicationunit.

In a simple embodiment, the base station simply transmits a stream ofpower down control values. This results in the transmit power beingreduced until the transmit power is minimised, which specifically maycorrespond to a substantially zero transmit power. The further powerdown control values will result in the transmit power being maintainedat the minimum level. In this embodiment, steps 205 and 207 areequivalent and comprise continuously transmitting power down controlvalues.

In more complex embodiments, step 207 comprises transmitting powercontrol values such that communication is still possible between thecommunication unit and the base station. Hence, the transmit power isreduced but not to a substantially zero transmit power level. Forexample, a power control similar to the first mode power control may beoperated but for a lower quality requirement. For example, a lower SIRmay be required resulting in lower transmit power but increased errorrates. However, this may in some communication systems be compensated byreducing the data rate of the communication. For example, an increasedforward error correcting scheme may be used.

Step 207 is followed by step 209 wherein it is determined whether thebase station 111 should remain in the reduced power mode or exit to thefirst mode of operation. If it is determined that the base station 11should remain in the reduced power mode, the method continues byiterating step 207. If it is determined that the base station 111 shouldexit the reduced power mode, the method continues in step 211.

In one embodiment, step 209 comprises determining that a duration of thereduced power mode exceeds a threshold. For example, a duration of thereduced power mode may be predetermined. Step 209 checks whether thecurrent duration of the reduced power mode exceeds this predeterminedvalue and if so proceeds to step 211. In this embodiment, the durationof the reduced power mode may be of a fixed duration. Hence, the basestation 111 enters a reduced power mode in response to detecting anunfavourable propagation characteristic and exits the reduced power modeafter a fixed duration. The fixed duration is preferably sufficientlylong to have a reasonably high probability of the propagationcharacteristics having changed. If not, the base station may detect thisand enter the reduced power mode again.

Preferably, the fixed duration is less than a data re-transmissioninterval associated with the communication between the communicationunit and the base station. Many communication systems comprisere-transmission schemes for re-transmitting lost data. For example, apacket data scheme may comprise re-transmitting any data packets thathave not been acknowledged by the receiving end. Hence, if the reducedpower mode is of a duration less than the data re-transmission intervalin which lost data packets may be re-transmitted, any loss of dataduring the reduced power mode may be compensated by retransmissions.

Alternatively or additionally, step 209 comprises determining that aquality characteristic of a data communication between the communicationunit and the base station is improving. As the transmit power of thecommunication unit reduces, it is expected that the error rate increasesand the SIR reduces accordingly. However, if the base station detectsthat either the error rate decreases or the SIR increases, this is anindication that the propagation characteristics have improved for thecommunication. Accordingly, communication is possible without causingexcessive interference and the reduced power mode is exited.

Alternatively or additionally, step 209 comprises determining that aninterference level is below a threshold. For example, the base stationmay determine that the total received interference level fills below alevel corresponding to full loading and that accordingly resource isavailable for continuing the communication with the communication unitin the normal mode of operation.

Alternatively or additionally, step 209 comprises determining that apropagation characteristic is below a threshold. Specifically, the basestation may continuously determine the propagation loss from knowledgeof the received signal level and the reduced transmit power of thecommunication unit. If the propagation loss decreases below a giventhreshold the communication may be continued without causing excessiveinterference and accordingly the reduced power mode is exited.

In step 211, the base station exits the reduced power mode bycommunicating power up power control data to the communication unit. Inthe preferred embodiment, the base station transmits power up powercontrol data until the transmit power of the communication unitcorresponds to a power level corresponding to the power level prior toentering the reduced power mode. Hence, the base station determines howmany power up control commands that need to be transmitted in order forthe transmit power to be equal to that before entering the reduced powermode. This will result in a high probability of the transmit power beingreturned to a suitable level.

In another embodiment, power up power control data is transmitted untilthe transmit power corresponds to a power level determined in responseto the quality parameter. Specifically, power up messages may betransmitted until a given SIR or error rate is achieved.

In one embodiment, the reduced power mode is used in resourceallocation. For example, the reduced power mode may be used forscheduling packet data traffic in the uplink direction.

In one such embodiment, scheduling occurs at the beginning of aTransmission Time Interval (TTI). Using the historical uplinktransmission information the probability distribution may be estimatedof the probability of an active communication unit transmitting a datapacket in a TTI. Further, the conditional transmission probability,p(k+1|k), of an active communication unit communicating during the nextTTI given that it has been busy for the past k TTIs may be determined.In addition an expected interference level may be determined for eachactive communication unit.

In the embodiment, the total expected interference level is determinedat the beginning of a TTI taken into account the probability oftransmission and the expected interference level if each activecommunication unit. Specifically, the total expected interference levelis determined as:${\hat{I}(k)} = {\sum\limits_{i \in {\Theta{(k)}}}{{p_{t}\left( {k + 1} \middle| k \right)}{I_{t}(k)}}}$where I_(i)(k) is the expected interference level of communication uniti and Θ(k) is the index set of all active communication units.

If Î(k) is greater than a given threshold I_(TH), then one or morecommunication units are entered into a reduced power mode. Otherwise,all communication units are allowed to proceed with normalcommunication.

If Î(k) is greater than the threshold I_(TH), the base station enters anreduced power mode for the communication unit with the highestprobability of transmission p_(i)(k+1|k). The total expectedinterference level is re-calculated taking this into account. Theprocess is repeated for additional communication units until the totalexpected interference level is below I_(TH).

In other embodiments, communication units may be selected for reducedpower mode operation in response to the interference contribution ofthat communication unit or in response to both the interferencecontribution and the probability of transmission

It will be appreciated that the embodiments described above have beenconcerned with uplink power control and controlling a transmit power ofthe remote communication units. However, it will be apparent that theinvention is equally applicable to downlink power control and thecontrol of a transmit power of a base station. In this case, thecontrolled transmit power is preferably the transmit power associatedwith the communication to the communication unit.

The above description has for clarity and brevity focussed on enteringreduced power mode for one communication unit. However, it will beapparent that the method may be repeated for a plurality or indeed allcommunication units of the cell or the communication system If themethod is continuously applied to all communication units served by thebase station, there will be a tendency to temporarily suspendcommunication units operating with unfavourable propagation conditionsand consequently causing a disproportional interference. Hence, thecommunication units will be biased towards communication when the causedinterference is low, and thus the capacity of the base station may beincreased significantly.

The invention can be implemented in any suitable form includinghardware, software, firmware or any combination of these. However,preferably, the invention is implemented as computer software running onone or more data processors and/or digital signal processors. Theelements and components of an embodiment of the invention may bephysically, functionally and logically implemented in any suitable way.Indeed the functionality may be implemented in a single unit, in aplurality of units or as part of other functional units. As such, theinvention may be implemented in a single unit or may be physically andfunctionally distributed between different units and processors.

Although the present invention has been described in connection with thepreferred embodiment, it is not intended to be limited to the specificform set forth herein. Rather, the scope of the present invention islimited only by the accompanying claims. In the claims, the termcomprising does not exclude the presence of other elements or steps.Furthermore, although individually listed, a plurality of means,elements or method steps may be implemented by e.g. a single unit orprocessor. Additionally, although individual features may be included indifferent claims, these may possibly be advantageously combined, and theinclusion in different claims does not imply that a combination offeatures is not feasible and/or advantageous. In addition, singularreferences do not exclude a plurality. Thus references to “a”, “an”,“first”, “second” etc do not preclude a plurality.

1. A method of power control for a transmitter in a cellularcommunication system comprising the steps of: determining power controldata in response to a quality parameter of a communication between abase station and a communication unit, and communicating the powercontrol data between the base station and the communication unit;entering a reduced power mode of operation by communicating power downpower control data between the base station and the communication unit;operating in the reduced power mode by communicating power control datacorresponding to a reduced transmit power level; and exiting the reducedpower mode by communicating power up power control data between the basestation and the communication unit.
 2. A method as claimed in claim 1wherein the power control is an uplink power control and the powercontrol data is transmitted from the base station to the communicationunit.
 3. A method as claimed in claim 1 wherein the power control is adownlink power control and the power control data is transmitted fromthe communication unit to the base station.
 4. A method as claimed inclaim 1 wherein the reduced transmit power level is substantially zero.5. A method as claimed in claim 1 wherein the power control datacommunicated in the reduced power mode is power down control values. 6.A method as claimed in claim 1 wherein the reduced transmit power levelallows a reduced data rate communication between the communication unitand the base station.
 7. A method as claimed in claim 1 wherein the stepof exiting comprises transmitting power up power control data until thetransmit power corresponds to a power level determined in response tothe quality parameter.
 8. A method as claimed in claim 1 wherein thestep of exiting comprises transmitting power up power control data untilthe transmit power corresponds to a power level corresponding to thepower level prior to entering the reduced power mode.
 9. A method asclaimed in claim 1 wherein a duration of the reduced power mode is lessthan a data re-transmission interval associated with the communicationbetween the communication unit and the base station.
 10. A method asclaimed in claim 1 further comprising the step of determining that aquality level of the communication between the communication unit andthe base station cannot be achieved, and in response entering thereduced power mode.
 11. A method as claimed in claim 1 furthercomprising the step of determining that a transmit power of thetransmitter exceeds a threshold and in response entering the reducedpower mode.
 12. A method as claimed in claim 1 further comprising thestep of determining that an interference level exceeds a threshold andin response entering the reduced power mode.
 13. A method as claimed inclaim 1 further comprising the step of determining that a propagationcharacteristic exceeds a threshold and in response entering the reducedpower mode.
 14. A method as claimed in claim 13 wherein the propagationcharacteristic is a path loss of a communication link supporting thecommunication between the communication unit and the base station.
 15. Amethod as claimed in claim 1 further comprising the step of determiningthat a duration of the reduced power mode exceeds a threshold and inresponse exiting the reduced power mode.
 16. A method as claimed inclaim 1 further comprising the step of determining that a qualitycharacteristic of a data communication between the communication unitand the base station is improving and in response exiting the reducedpower mode.
 17. A method as claimed in claim 1 further comprising thestep of determining that an interference level is below a threshold andin response exiting the reduced power mode.
 18. A method as claimed inclaim 1 further comprising the step of determining that a propagationcharacteristic is below a threshold and in response exiting the reducedpower mode.
 19. A method as claimed in claim 17 wherein the propagationcharacteristic is a path loss of a communication link supporting thecommunication between the communication unit and the base station.
 20. Amethod as claimed in claim 1 further comprising the steps of:determining an expected interference level for a plurality ofcommunication units including the communication unit; determining atotal expected interference level; and entering the communication unitinto the reduced power mode if the total expected interference levelexceeds a threshold. 21-23. (canceled)
 24. An apparatus for powercontrol for a transmitter in a cellular communication system, theapparatus comprising: means for determining power control data inresponse to a quality parameter of a communication between a basestation and a communication unit; means for communicating the powercontrol data between the base station and the communication unit; meansfor entering a reduced power mode of operation by communicating powerdown power control data between the base station and the communicationunit; means for operating in the reduced power mode by communicatingpower control data corresponding to a reduced transmit power level; andmeans for exiting the reduced power mode by communicating power up powercontrol data between the base station and the communication unit.